Studies already published, from this and other laboratories, strongly suggest that in some circumstances the muscles of ventilation can become fatigued. The proposed research addresses the following specific questions that are currently unanswered: 1. What is the temporal relationship between the electrophysiologic and mechanical manisfestations of respiratory muscle fatigue? 2. Can the phenomenon be detected reliably by respiratory electromyograms (EMGs) recorded from surface electrodes applied over the chest wall? 3. If so, what is the most reliable analytical technique? 4. What factors either contribute to, or retard the onset of, respiratory muscle fatigue? 5. What is its prevalence in disease? We propose using the techniques for power spectral density analysis already developed in the laboratory to detect spectral shifts in the EMG signal, and correlating these changes initially with direct measurements of respiratory pressures over time. Studies will be performed in normal humans and in an animal model (cat diaphragm) that has been found to resemble the human muscle. Particular care will be taken to characterize any artifacts that may interfere with interpretation of the experimental data. The ultimate aim of the research is to establish a rapid-response monitoring system for clinical use; progress towards this goal, and future plans in this respect, are described.